APOE genotype is the strongest genetic risk factor for late-onset Alzheimer?s disease (AD). A complete understanding of its role in AD pathogenesis remains unclear. In addition to the effects of apoE on A? clearance and aggregation, it is also likely to also influence AD via other mechanisms. Over the last 4.5 years of this grant, we have utilized a mouse model that develops tauopathy and neurodegeneration (P301S Tau Tg mice) and found that all human isoforms of apoE significantly enhance tau-mediated brain volume/neuronal loss and disease associated microglial and astrocyte response with apoE4 having the strongest effects. Interestingly, tau-mediated neurodegeneration and the enhanced innate immune response was strongly attenuated in the absence of apoE. Using a cell culture model, we also found that glial-derived apoE, particularly apoE4, could exacerbate neuronal damage, an effect that appears to involve apoE?s ability to enhance inflammation. In preliminary data, we have now found that if we overexpress the low density lipoprotein receptor (LDLR) in the brain of P301S Tau Tg mice, we can markedly lower apoE and attenuate brain atrophy, decrease p-tau levels, and decrease the disease associated microglial-response. We also found that cultured astrocytes secrete apoE in cholesterol/phospholipid rich HDL-like particles whereas cultured microglia secrete apoE in small, poorly lipidated particles. While these findings suggest important roles for apoE in modulating tau-mediated neurodegeneration, there are a number of important unanswered questions to address to better understand these effects. They include 1) Are astrocytes or microglia the main source of apoE that mediates these effects? 2) In addition to effects on A?, does astrocyte vs. microglial apoE in an isoform-specific fashion affect A?-induced tau seeding and spreading? 3) How do astrocyte and microglial- derived apoE particles differ, what is their structure and how does it differ by isoform, and do these differences result in different biological effects? 4) How and what is the source of apoE that influences the brain?s innate immune response to influence tau-mediated neurodegeneration? These questions lead us to hypothesize that apoE, particularly apoE4, influences the brain?s innate immune response to exacerbate tau- mediated neurodegeneration and A?-induced tau seeding/spreading. We propose these Aims: Aim 1: To determine whether apoE produced by astrocytes or microglia influences tau-mediated neurodegeneration using newly generated conditional apoE KI mice. Aim 2: To determine whether apoE produced by astrocytes or microglia influences human AD tau seeding/spreading +/- A? as well as CSF and plasma levels of p-tau, tau, A?, and apoE using a well characterized mouse model that develops A? amyloidosis APPNL-F KI mice. Aim 3: To characterize apoE particles produced by astrocytes and microglia both in vitro and in vivo via lipidomics and cryoEM. We will utilize a variety of techniques including cryoEM and lipidomics to characterize microglial and astrocyte-produced apoE particles.